Control valves are used extensively in different types of industries to control physical conditions like flow, pressure, temperature and liquid level. Controlling such conditions is very important in oil & gas and manufacturing industries to maintain a steady flow of materials within the safety limits. This is done by opening or closing the control valves, either completely or partially to regulate the flow of fluids, in response of the sensors. The sensors sense and measure the conditions like pressure and temperature; and a microcontroller compares their magnitude to the specified value. After comparison it sends appropriate controlling signals to open or close the control valve.
Earlier control valves were opened or closed manually, but these days all most every industry use automatic control valves. Such valves are regulated by electrical, hydraulic, and pneumatic or electro hydraulic actuators, in response to electric or pneumatic control signals. Traditionally 3-15 psi (0.2-1.0 bar) control signal was used in the industry for 0-10V for HVAC systems, but now a day's 4-20 mA signals are being used in these systems. Today several types of control valves are used in the industry, which vary in design and construction. Typically control valves have three main parts: valve's actuator, valve's positioners and valve's body.
Valve's actuator: It is used to automatically open or close the valve, according to the controlling signal to vary the flow of fluids. Depending on the source of power of the actuator, there are four main types of actuators: electrical actuators, hydraulic, electro hydraulic actuators and pneumatic actuators. Electrical actuators have motors which are driven by an electrical current or voltage supply. Such actuators are easy to control, as electric signals can be easily moderated by micro controllers and amplifiers. Hydraulic and pneumatic actuators are very similar in structure, and contain a piston and cylinder arrangement. The reciprocating motion of piston is obtained by pressurizing and de-pressurizing the cylinder. In case of hydraulic actuator, the cylinder is filled with water, while in pneumatic ones, it is filled with air. Electro hydraulic cylinders are a combination of electric and hydraulic cylinders, and can drive heavier loads, and at the same time are controlled easily by electric signals.
Valve's positioners: They are used to accurately determine which valve is to be opened or closed. For better positioning, digital valve positioners are being used in the industry, these days.
Valve's body: Mostly bodies of the control valves have a sliding-stem globe and angle valves. Such a design has rugged construction which makes it suitable for several versatile applications.